Centrifuge rotors have typically been constructed from metal forgings. Metals are known to be isotropic in their strength of material properties. Their resistance in tension, compression, shear, and bending is for the most part uniform in all differing directions of possible stress. Taking the case of titanium, the resistance is roughly uniform--about 250,000 pounds per square inch.
Centrifuge rotors built of resin reinforced composite fibers are known. Composite fibers are anisotropic in their strength of material properties. Taking the case of titanium, composite fibers have much greater tensile strength (up to five times or 1,250,000 psi) and are much lighter (about one-third the weight of titanium). The resistance of composite fibers in compression, shear, and bending is another matter. The fibers have virtually no resistance to these forces--especially when the forces are applied length-wise of an individual fiber.
The building of composite rotors--especially centrifuge rotors--is known. In the most common prior art constructions, resin impregnated fiber tape and/or fabric is cut in discrete discs. The discrete discs are stacked one upon another, with the discs disposed normal to the spin axis of the ultimately constructed rotor. Typically, as each discrete disc is stacked, the angularity of the fiber of the discrete disc is varied with respect to the angularity of the fiber in the disc on to which it is stacked. This stacking process with varied angular alignment continues until a cylindrical rotor body is fabricated.
In another composite rotor construction technique, fibers have been compression molded. Specifically, fibers are placed with a mold in so-called sheet molding compound, having a mixture of fiber and resin. The sheet molding compound has randomly disposed fibers all confined within a plane. The fibers are then compressed and cured. Although the fibers undertake some migration during the compression molding, the fibers remain largely with their horizontal disposition.
In both of these techniques, the fibers are not optimally aligned so that their superior isotropic properties in tension can be utilized. A brief discussion of the dynamic forces extant upon a spinning centrifuge rotor can emphasize this point.
Taking the case of a spinning centrifuge rotor, the rotor resists the centrifugal forces radially and circumferential. In radial resistance, the rotor uses the continuity of material across the spin axis. In circumferential resistance, the material of the rotor acts in hoop tension around the circumference of the rotor.
Going back to the two known constructions of composite centrifuge rotors, it will be seen that many of the fibers are placed in dispositions other than radial or circumferential. In the use of composite discs, it will be understood that most fibers of the disc will not have a radial alignment. Further, none of the fibers of the discs will have circumferential alignment.
Nevertheless, the resulting rotors are remarkably strong in resisting centrifugal stress imposed parallel to the planes of the disc layers. The varied angular alignment of fibers in the discrete discs making up the rotor imparts to the disc a resistance which I term as "quasi-isotropic" resistance. The resin impregnated fiber of each layer which happens to be optimally disposed (usually radially disposed for resistance to centrifugal stresses) imparts to the fibers of adjacent layers resistance to centrifugally generated stress. Thus, the fibers when fastened with resin in effect reinforce each other through the resin, even though the fibers being reinforced are not optimally aligned (either radially or circumferentially) to resist the stress.